The invention is based on a priority application EP 00 440 275.6 which is hereby incorporated by reference.
The invention concerns a device for erasing overhead modulation of an optical signal, wherein said optical signal is injected into an optical device able to modulate the optical signal (for example a semi-conductor optical amplifier driver by a bias control).
The term xe2x80x9coverhead modulationxe2x80x9d as used herein comprises cases, where a pilot tone (which itself is not modulated with varying information) is used for identifying or other purposes, as well as such cases where the overhead modulation carries a preferably binary information. In the first case, the pilot tone for one specific optical channel is a signal with a pure frequency modulation. In the second case, the overhead modulation is modulated in order to carry the information in +he form of e.g. bits of the value 0 and 1.
The following concept can be applied to any optical device able to modulate an optical signal (not SOA only).
It is known to erase a pilot tone in that way that the optical signal output from the semi-conductor optical amplifier is sensed, that the detected modulation is inverted, fed back to the control input of the semi-conductor optical amplifier, and that the residual signal of the pilot tone is used to control the inverting electronic amplifier in such a way that the residual signal becomes (as far as possible) zero. This solution needs access to the optical path at the output side of the semi-conductor optical amplifier, and needs, of course, also access to the control terminal of the semi-conductor optical amplifier. This known solution cannot be used in the case of an integrated device for which we do not have access to the output of the SOA. Typically for optical wavelength converters based on a Mach-Zenhder Interferometer structure it is not possible to detect the optical signal between two SOAs.
The invention provides a method which does not need access to the output side of the optical amplifier. The invention, starting with the device stated above, is characterised in that said bias control means applies to said amplifier a compensation voltage presenting a modulation out of phase with respect to said overhead modulation, and in that modulation index of said compensation voltage is (substantially) proportional to the modulation index of said optical signal.
Instead of a closed loop control as used for the above mentioned erasing of a pilot tone, the invention uses an open loop control; therefore, checking of the possible presence of a residual signal of the overhead modulation at the output side of the optical amplifier is not needed. (Nevertheless, for example for testing purposes, it might by advisable to detect the residual signal in any appropriate manner).
The invention needs to sense only the optical signal at the input side of the semi-conductor optical amplifier. From the sensed signal, the wave form of the overhead modulation including the binary information, if any, is gathered, the corresponding signal is inverted, amplified by a factor which takes into account amplification characteristics of the semi-conductor optical amplifier, to which the optical signal is being fed. The such amplified signal is fed forward to a control input of said semi-conductor optical amplifier. Refreshing the original present overhead modulation (or pilot tone) or updating other information may be accomplished by adding the new overhead modulation to the signal for erasing the old overhead modulation and feeding the two signals to the semi-conductor optical amplifier.
The above mentioned amplifying characteristics of the SEMI-CONDUCTOR OPTICAL AMPLIFIER are the response of the optical gain of the SEMI-CONDUCTOR OPTICAL AMPLIFIER versus the electronic control (20). For low amplitude electrical modulations, the first order of this characteristic is linear.
In an embodiment of the invention, the overhead modulation comprises or consists of a pilot tone.
In an embodiment of the invention, the bias control means comprises an inverting electrical amplifier with gain control, such that
g=V0/P0,
where g is the gain, P0 the input voltage of the electrical amplifier corresponding to the mean optical power of the optical signal, and V0 is the mean bias voltage applied to the semi-conductor optical amplifier.
In an embodiment of the invention, the compensation voltage is out of phase by generally 180xc2x0 with respect to the optical signal.
In an embodiment of the invention, the overhead modulation is sensed from the optical signal at the side of the optical input of the semi-conductor optical amplifier.
In an embodiment of the invention, the bias control means uses the sensed optical signal and, as parameter, known amplification characteristics of the semi-conductor optical amplifier.
The invention also includes a system comprising a device according to the invention, wherein in order to actualise overhead modulation of an optical signal output from the semi-conductor optical amplifier, a new overhead modulation signal is fed to the input for the bias control signal of the semi-conductor optical amplifier.
In an embodiment of the invention, the new overhead signal is added to the signal produced by the bias control means.